1. Field of the Invention
This invention relates to plotters used for scientific and industrial purposes to record data, such as time chart recorders, and specifically to the means used to control such plotters.
2. Description of the Prior Art
Plotters of various kinds are well known in the art. One kind of plotter is used to record data as output from scientific and industrial instrumentation. Typically as shown schematically in FIG. 1, this kind of plotter 2 operates as a strip chart recorder, with a roll of paper 4 (or other media) fed through the plotter 2, and a carriage 6 including a printing device such as a print cartridge moving back and forth on a guide rail along a Y axis as shown relative to the medium 4. Thus the X-axis as shown typically represents the first variable, time, and the second axis is the Y-axis, as defined by the direction of carriage 6 movement, and typically represents a second variable such as a signal.
One version of this type of plotter is ink jet printer-plotters which plot and print by means of ink dots ejected through orifices of a print cartridge which is mounted in the carriage 6. The best known type of print cartridge is the Hewlett Packard thermal ink jet cartridge. Data is output to the printer-plotter 2 on channel 7 from instrumentation 8 or a computer. The printer 2 typically has a built-in microcontroller 10, typically an 8 bit microprocessor. A computer program (not shown) is conventionally installed in ROM 12 in the microcontroller 10 so as to control the printer-plotter 2. The actual plotted line 14 is shown as plotted on medium 4.
The printer-plotter 2, as is typical of this type of device, advances the medium 4 (e.g., paper) in the X direction through the printer-plotter 2 in steps, not continually. The paper is advanced typically by means of a stepper motor. When doing real time data plotting, typically the data points are sent from the system CPU 8 (i.e., the instrumentation or computer) to the plotter every tenth of a second, or 600 times per minute. The chart speeds (i.e., the speed of the medium 4 through the plotter) available range typically from one tenth of a centimeter per minute to 20 centimeters per minute. Thus the medium advances for example at 630 steps to the inch on the time (X direction) axis or 248 steps per centimeter. Since data input points are provided at, for example, 600 times per minute, (600 points/minute).div.(248 steps/cm) is equal to 2.42 centimeters per minute. This means that for this example any time the medium is moving faster than 2.42 centimeters per minute, the medium will take at least one step in the time axis for each data point. When moving slower than that speed there will be two or more data point times for some time steps. If the printer is advancing the medium at its slowest speed, 0.1 centimeter per minute in the example, there will be twenty-four data points plotted for one step in the time axis. Thus one step in the time axis will have twenty-four data points plotted on one time increment. Low level noise motion on the signal axis can cause repeated excursions of the carriage over the same line even if no new data is plotted, providing undesirably twenty-four times as much ink on the medium as needed. This gives a very dark plot line, the ink of which tends to run and so undesirably produces an uneven and smeared plot line as shown in FIG. 2A.